The use of helium diffraction patterns to study desorption processes is explored as a novel extension to traditional methods based on helium
specular reflection. The sample, cyclooctatetraene adsorbed on Cu(111), provides a rich but complex structure. The modulation of cyclooctatetraene by Cu(111) is manifested as a convolution in the diffraction pattern, displaying an averaged super-cell symmetry of (7 √3 × 7 √3)R30◦. The adlayer expands during isothermal desorption, and the change in lattice constant provides a direct measure of the coverage as a function of time. We find a desorption energy of 0:96 ± 0:01 eV at saturation of the first
layer, and an upper limit of 1:62 ± 0:07 eV for isolated molecules. These values, and details of the assigned structure, indicate chemisorbed
molecules with a planar conformation.For financial support, N. Avidor gratefully acknowledges the Blavatnik Foundation, J.A. Lau acknowledges the ICASEC, P.S.M. Townsend acknowledges the UK EPSRC, I. Calvo-Almaz´an acknowledges the Ram´on Areces Foundation, BJ Hinch acknowledges the NSF (CHE-1565673). The authors thank G. Alexandrowicz for useful discussions.
EPSRC (EP/E004962/1)
EPSRC (1363145
